Mercury levels in Pacific fish likely to rise in coming decades

University of Michigan researchers and their University of Hawaii colleagues say they’ve solved the longstanding mystery of how mercury gets into open-ocean fish, and their findings suggest that levels of the toxin in Pacific Ocean fish will likely rise in coming decades. Using isotopic measurement techniques developed at the University of Michigan, the researchers determined that up to 80% of the toxic form of mercury, called methylmercury, found in the tissues of deep-feeding North Pacific Ocean fish is produced deep in the ocean, most likely by bacteria clinging to sinking bits of organic matter.

The study, published in Nature Geoscience, also confirmed that the mercury found in Pacific fish near Hawaii likely traveled through the air for thousands of miles before being deposited on the ocean surface in rainfall, said co-author and University of Michigan Environmental Scientist Joel Blum. The North Pacific fisheries are downwind from rapidly industrializing nations such as China and India that are increasingly reliant on coal-burning power plants, a major source of mercury pollution.

“This study reinforces the links between mercury emitted from Asian countries and the fish that we catch off Hawaii and consume in this country,” said Blum. “The implications are that if we’re going to effectively reduce the mercury concentrations in open-ocean fish, we’re going to have to reduce global emissions of mercury, including emissions from places like China and India. Cleaning up our own shorelines is not going to be enough. This is a global atmospheric problem.”

The researchers sampled fish at various depths in Hawaiian waters, measured the total amount of mercury in their muscle tissues, and determined their position in the marine food web. Then, they showed how and where methylation occurs in the open ocean and explained the previously observed increases in the mercury concentration of predatory fish with depth. They found that while methylation occurs in well-lit near-surface waters, sunlight destroys up to 80% of the methylmercury formed there, through a process called photochemical degradation.

“The crystal-clear waters surrounding Hawaii and the unique information that we had about the depths at which our local fish feed allowed us to clearly identify both the photochemical degradation of methylmercury at surface levels and the microbial production of methylmercury from inorganic mercury in deeper waters,” said Brian Popp, Professor of Geology and Geophysics at the University of Hawaii at Manoa and co-author of the study.

In addition, the isotopic composition of the mercury found in the fish tissues was “a nearly perfect match” with the chemical signature of mercury in the atmosphere known to travel long distances, far from pollution sources such as coal-burning power plants, Blum said.

That finding confirms an idea that was long-suspected but previously unsupported by hard evidence: “These results strongly support the hypothesis that long-range transport of mercury deposited to the ocean surface is ultimately what’s ending up in these fish,” said Blum. The nine species of fish used in the study, listed from shallowest- to deepest-feeding, are flying fish, mahi-mahi, yellowfin tuna, skipjack tuna, moonfish (opah), bigeye tuna, swordfish, and two species of lantern fish.